For filtering particulates from air in a vacuum cleaning system, the basic goal is to remove particulate matter from the air passing through the system while maintaining an efficient air flow. Mechanical filters are frequently used, but can quickly become covered by filtered material and saturated with a cake of filtered material, reducing the efficiency of the air flow through the filter.
Cyclone type vacuum cleaners are known in the art, as disclosed, for example, in U.S. Pat. No. 2,233,167 to Holm Hansen. This system functions as a centrifugal separator wherein incoming air is formed into a whirling annulus or cyclone. The solid particulates in the air, by virtue of their inertia, tend to move to and impinge the outside wall where they impact and settle into a collector or dust receptacle. In the Holm-Hansen system, water in the collector is driven by the whirling air into a wave the travels around the dust-receptacle, wetting and washing the wall. These systems work well to "rough clean" the air before extraction of the finest particles in the air by a mechanical filtration system. These systems work well, but unfortunately such a single-stage cyclone system as typically used in vacuum systems fails to remove enough of the mid-range particulate to prevent premature caking of the final solid filter. Thus the final filter must be frequently replaced, and that filter is inefficiently used. The final filter is designed to remove the finest particles, but becomes quickly clogged and inoperative by large and mid-range particles. This is especially problematic when the mechanical filter uses an ultrafine material, such as an expensive HEPA medium.
Another problem with cyclonic systems, like the Holm-Hansen system, that the particle removal, which depends upon the swirling of water by the air flow along the outer wall of the cyclone container, the removal efficiency often depends on the amount of liquid and level of liquid in the system. Usually a relatively small amount of liquid is used to maintain the optimum action, but the liquid quickly becomes over-burdened with particulate material, reducing its effectiveness. In addition, when such a system is used as a "wet-vacuum," such as to remove water from a flooded carpet, the cyclone fluid chamber becomes over-filled with water and effective particulate entrapment stops.